You should be able to setup a default inspection rule to block this... the ASA has predefined rules in the ASDM that are designed to block certain kinds of traffic such as this and other things like bittorrent and limewire... you just need to enable them.
If you go to the configuration tab in the ASDM and the go to firewall >> advanced you will find the inspection section... it should be fairly easy to setup blocking as the different items are listed by name.
Let me know if you have any questions! Cheers!

Oh - check this out - here is a screenshot from my ASDM - there is a default rule for blocking MSN Messenger, Yahoo Messenger, and lots of other things.
Just activate these policies. No need for booobs or devangshroff's commands. Lol.
Cheers! Let me know if you have any questions!

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